The coating of electrically conductive substrates by electrophoretic deposition processes, or electrocoat or ecoat, is a well-known and important industrial process. The electrodeposition of primers to metal substrates used for automotive bodies is widely used in the automotive industry. In the electrodeposition of primers, a conductive article, typically metal, is immersed in a bath of a primer composition of an aqueous emulsion of film forming polymer and acts as one electrode in the electrodeposition process. An electric current is passed between the article and acts as a counter-electrode in electrical contact with the aqueous emulsion, until a desired thickness of coating is deposited on the article. In a cathodic electrocoating process, the article to be coated is the cathode and the counter-electrode is the anode. In an anodic process, the article to be coated is the anode.
In traditional automotive processes, the electrodeposited first layer is typically followed by spray application of a second coating composition, often a primer surfacer type coating or the like. It would be advantageous if the second coating could be electrodeposited. The electrodeposition process offers several advantages over spray application processes. For example, electrocoat processes typically have lower solvent emissions, reduced operating costs, higher effective paint utilization, and more uniform coating thickness. However, the surface to be coated must be conductive for electrodeposition to occur. Eventually, the electrocoated surface becomes nonconductive due to the insulative build up of the polymeric film. The insulating nature of the polymeric coating ensures a uniform film build and makes the process self limiting.
The prior art has thus sought a multilayer coating composition wherein at least two layers are applied by electrophoretic deposition processes, preferably wherein a first electrodeposited layer is subsequently overcoated with a second electrodeposited layer.
Streitberger, Beck and Guder disclosed multilayer electrocoatings in which the first layer was modified with conducting carbon blacks. (Proc. XlXth FATIPEC Congress, Aachem 1988, Vol. 2, 177-89) The epoxy based cathodically electrodeposited layer had specific resistance values which were greater than the unpigmented films. Additions of conducting black maintained the self-limiting nature of the electrocoating process. The imparted conductivity made the overcoating possible in a second electrocoat process.
U.S. Pat. No. 4,968,399 discloses a multiple electrocoating process comprising coating once or more on a substrate a first electrocoating composition, coating a second electrocoating composition on the uncured first electrocoated composition and then curing all electrodeposited coatings. The first electrodeposited coating is an aqueous dispersion containing micro gel particles having an electric charge that is specially prepared. The second electrocoat composition comprises an anionic or cationic film forming aqueous resin (C) and a thermosetting agent (D) that is self-crosslinked with said aqueous resin (C).
However, this method appears to be disadvantageous due to the use of the conductive micro gel particles.
U.S. Pat. No. 5,104,507 discloses an electrodeposition method wherein a conductive substrate is first coated by means of an anodic electrodeposition process, then subjected to thermal curing, and then subjected to a cathodic electrodeposition process. The anodic electrodeposition process is carried out until the coating insulates the substrate at which point the coating process stops. The curing step lowers the dielectric strength of the anodically deposited coating, thereby allowing the substrate to accept a cathodically deposited topcoat. The patent expressly teaches that the advantageous results of the disclosed invention cannot be obtained via the use of a cured cathodically electrodeposited first coating overcoated with an anodically electrodeposited second coating.
U.S. Pat. No. 5,203,975 discloses an electrocoating process in which a layer of a clear cathodic electrocoating composition is electrocoated over a layer of a conductive composition and cured. The conductive composition is an electrically conductive cathodic electrocoating composition containing film forming binder and pigment in a pigment to binder ratio of about 1:100 to 100:100, wherein the pigment comprises an electrically conductive pigment of silica which is either amorphous silica or a silica containing material, the silica being in association with a two dimensional network of antimony containing tin oxide crystallites in which the antimony content ranges from about 1-30% by weight of the tin oxide. However, such particular silica containing materials are commercially disadvantageous.
U.S. Pat. No. 5,223,106 discloses an electrophoretic coatable sealant composition for use in assembling automotive bodies comprising an adhesive material selected from the group consisting of epoxys, urethanes, epoxyurethane hybrids, acrylics, epoxy-acrylic hybrids, polyvinylchlorides and mixtures thereof and a conductive filler in an amount such that the sealant composition will accept electrophoretic deposition of a primer composition yet preserve the adhesive properties of the sealant composition.
Finally, U.S. Pat. No. 5,275,707 discloses a method of coating a metal article by forming a first electrodeposition coating layer having varistor properties, and then forming a second electrodeposition coating layer on said first electrodeposition coating layer by an electrodeposition coating method by use of an anionic or cationic electrodeposition coating while applying a voltage exceeding the varistor voltage.
Thus, the prior art has failed to provide a process for making a multilayer electrodeposited coating composition having advantageous application and performance properties.
It is thus an object of the invention to provide a multilayer coating composition wherein at least two successively applied layers are applied by electrophoretic deposition processes and the multilayer coating composition has advantageous properties.